Approximate Date
of Proposed Public Offering: As soon as practicable following effective date.

It is proposed that this filing will become effective
(check appropriate box)

x

immediately upon filing pursuant to paragraph (b)

¨

on (date) pursuant to paragraph (b)

¨

60 days after filing pursuant to paragraph (a)(1)

¨

on (date) pursuant to paragraph (a)(1)

¨

75 days after filing pursuant to paragraph (a)(2)

¨

on (date) pursuant to paragraph (a)(2) of rule 485.

If appropriate, check the following box:

¨ this post-effective amendment designates a new effective date
for a previously filed post-effective amendment.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, as amended, the Registrant certifies that it meets all the requirements for effectiveness of this
Registration Statement under Rule 485(b) under the Securities Act and has duly caused this Post-Effective Amendment No. 159/160 to its Registration Statement on Form N-1A to be signed on its behalf by the undersigned, hereunto duly authorized
in the City of Austin, the State of Texas on this 20th day of March, 2013.

DFA INVESTMENT DIMENSIONS GROUP INC.

(Registrant)

By:

/s/ David G. Booth*

David G. Booth, President

(Signature and Title)

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed
below by the following persons in the capacities and on the date indicated: